The invention relates to a method for filling a cavity of a mold for producing a molding from a melt, in particular a cavity of an injection-molding machine, in which the melt is introduced into the cavity under pressure and is placed under a holding pressure toward the end or at the end of its flow path.
To influence the uniformity and good quality of injection-molded products, it is known to determine the temperature of the melt. By way of example, in accordance with JP 61255825, the temperature of the melt or the currently measured temperature of the mold is compared with a multiplicity of predetermined temperature values.
During injection molding or also, for example, during die casting, the cavity in a mold or die is filled with a melt, in particular comprising plastic, metal or a ceramic. This filling operation continues until the cavity has been filled, after which the process is switched over to what is known as the holding-pressure phase, in which in particular shrinkage of the material in the cavity is also compensated for.
The determination of the changeover time from the introduction phase to the holding-pressure phase is an important factor. Manual optimization is difficult and time-consuming, and consequently is only seldom carried out correctly in practice.
A fixed changeover from the injection operation to the holding-pressure operation cannot react to process-induced fluctuations, for example in the viscosity, which in turn leads to a considerable fluctuation in the quality of the moldings.
By way of example, EP 0 707 936 A2 or U.S. Pat. No. 5,993,704 describes methods for determining the changeover time when producing an injection molding. These methods are used to automatically determine the time of volumetric filling in the mold cavity.
The existing methods are generally based on measuring the internal pressure. By way of example, the “kink” between the introduction phase and the holding-pressure phase, which results automatically, is determined with the aid of artificial intelligence.
In practice, however, it has been found that the calculation times in some cases result in excessive delays, which automatically entail pressure peaks and stresses in the molding. Moreover, it is impossible to universally cover all applications.
In a further method, the pressure differences between two mold internal-pressure sensors are determined and the profile of these differences over the course of time is analyzed. A sudden drop in the signal (=kink detection) means that volumetric filling has been detected.
Both the methods described above require relatively expensive sensor means and electronics and are not reliable processes.
DE 198 03 352 A has disclosed a method for determining the time of a pressure rise in a mold, with the process being switched over from the injection pressure to the holding pressure as a result of the pressure rise. In this case, the pressure rise is determined not by means of pressure sensors but rather by means of temperature sensors.
The present invention is based on the object of developing a method of the type described above which makes it possible to switch over to the holding-pressure phase in a simple and cost-effective way yet nevertheless with a relatively high level of accuracy.